The invention relates in general to transducer positioning in a magnetic data storage system and, more particularly, to compensation for repetitive run-out (RRO) created by a servo track writer (STW) in a magnetic data storage system.
A disk drive is a data storage device that stores digital data in tracks on the surface of a data storage disk. Data is read from or written to a track of the disk using a transducer that is held close to the track while the disk spins about its center at a substantially constant angular velocity. To properly locate the transducer near the desired track during a read or write operation, a closed-loop servo scheme is generally implemented that uses servo data read from the disk surface to align the transducer with the desired track. The servo data is generally written to the disk using a servo track writer (STW).
In an ideal disk drive system, the tracks of the data storage disk are non-perturbed circles situated about the center of the disk. As such, each of these ideal tracks includes a track centerline that is located at a known constant radius from the disk center. In an actual system, however, it is difficult to write non-perturbed circular tracks to the data storage disk. That is, problems, such as vibration, bearing defects, inaccuracies in the STW and disk clamp slippage can result in tracks that are written differently from the ideal non-perturbed circular track shape. Positioning errors created by the perturbed nature of these tracks are known as written-in repetitive runout (STW_RRO). The perturbed shape of these tracks complicates the transducer positioning function during read and write operations because the servo system needs to continuously reposition the transducer during track following to keep up with the constantly changing radius of the track centerline with respect to the center of the spinning disk. Furthermore, the perturbed shape of the these tracks can result in problems such as track squeeze and track misregistration errors during read and write operations.
In certain conventional systems, as will be understood by those skilled in the art, the STW is used to directly measure the STW_RRO for each track of a disk so that compensation values may be generated and used to position the transducer along an ideal track centerline. In such systems, the STW must measure the STW_RRO of each track of a disk one track at a time. Because (1) a typical disk drive has two or more disks (or four or more disk surfaces), (2) a typical disk contains over 20,000 tracks per inch (TPI), and (3) typical disk rotation speeds are around 7200 revolutions per minute (RPM), the STW could be busy for several hours in measuring the STW_RRO for one disk drive. The values of the STW_RRO for each track (or section of track) are then stored on the disk for use during transducer positioning. For an example of a disk drive system that is similar to the above described system, reference is made to U.S. Pat. No. 4,412,165 to Case et al. entitled xe2x80x9cSampled Servo Position Control System,xe2x80x9d which is incorporated herein by reference.
As is well-known in the art, STW""s are very expensive and, therefore, only a limited number of STW""s are available at a disk drive manufacturing facility. Accordingly, if the STW""s are tied up for extended periods of time in measuring the STW_RRO for each disk drive, the manufacturing throughput and efficiency of a manufacturing facility will be dramatically decreased.
In certain other conventional systems, as will be understood by those skilled in the art, the STW_RRO can be obtained if the error transfer function that describes the reaction of the servo control system to an input is known. In particular, the STW_RRO values can be determined by convolving the position error of the transducer head due to repeatable runout (PES_RRO) with the inverse impulse response. According to this conventional system, the error transfer function is determined from measured frequency response data or by computer modeling. In the case of using measured frequency response data, a discrete rational non-minimum phase polynomial transfer function is obtained by performing a least squares fit of the measured frequency data. For a model-based system, the error transfer function is directly calculated. The inverse error transfer function is determined by swapping the numerator and denominator of the discrete rational non-minimum phase polynomial error transfer function. Using the Z-transform, the non-causal impulse response is obtained from the inverse error transfer function. The STW_RRO values can then be determined by convolving measured PES_RRO values with the non-causal impulse response. Such a system is disclosed in U.S. Pat. No. 6,115,203, issued Sep. 5, 2000. the disclosure of which is incorporated herein by reference, and which is assigned to the assignee of the present invention.
However, the model-based non-causal impulse response obtained by the above-described system is inaccurate. In addition, the process of fitting a linear least squares finite order model to measured frequency response data introduces errors that carry through the calculations. Furthermore, the modeling of this transfer function is performed in non-real time. Accordingly, it is impractical to develop a non-causal impulse response of the inverse transfer function for an individual disk drive. Because of these limitations, the method of determining a non-causal impulse response from measured frequency data is not completely effective at canceling repeatable runout in a hard disk drive.
Therefore, it would be advantageous if a system were provided to compensate for STW_RRO without requiring the use of a STW to determine the STW_RRO. In addition, it would be advantageous if a system were provided to compensate for STW_RRO in a drive by utilizing that drive""s actual impulse response. Furthermore, it would be advantageous to provide a system that could be implemented in a high volume production environment.
The invention relates to a disk drive transducer positioning system and method for implementing the same that is capable of canceling written-in repetitive runout in substantially real time. Using the system, the transducer of the disk drive will follow a substantially non-perturbed circular path over the disk even though the written track is perturbed as compared with an ideal track. The system provides a significant improvement in at least track misregistration, write fault performance and seek settling time over disk drives that do not include the system. Furthermore, the present invention improves the efficiency of disk drive manufacturing facilities. The system is of particular benefit in disk drives having a relatively high track density.
In accordance with the invention, a disk drive system is disclosed. In one embodiment, the disk drive system comprises a data storage disk having one or more tracks. Each of the tracks have an ideal shape and an actual written shape. The disk drive further includes a means for estimating the actual written shape of the track. The means used does not include a STW. Rather, the means uses the disk drive containing the data storage disk.
In accordance with one embodiment of the present invention, a method and apparatus are provided in which embedded runout correction values are obtained by convolving PES_RRO values with the inverse impulse response of the servo system of the disk drive. In particular, according to the method and apparatus of the present invention, the inverse error impulse response for each transducer head in a hard disk drive is determined from a measurement of the impulse response to each transducer head. Accordingly, the method and an apparatus of the present invention provide a numerical impulse response function for a particular transducer head in a particular hard disk drive.
According to an embodiment of the present invention, the inverse impulse response is determined by measuring (in the time domain) the response of a servo control system associated with a transducer head to an impulse signal or function written to a track on the disk or otherwise provided to the servo control system. The error transfer function may then be obtained by transforming the measured impulse response to the frequency domain. According to one embodiment of the present invention, the transformation of the impulse response of the transducer head is performed using a Discrete Fourier Transform. The reciprocal of the resulting error transfer function may then be taken. The reciprocal error transfer function response may then be transformed back into the time domain to obtain the inverse impulse response of the servo system for the transducer head. According to one embodiment of the present invention, the transformation to the time domain is accomplished by using an inverse Discrete Fourier Transform. The runout correction values for a track are then calculated by convolving the position error due to repeatable runout with the inverse impulse response for the transducer head. According to another embodiment of the present invention, an inverse impulse response of a transducer head is obtained by introducing an impulse to the transducer head from a track located towards an outer diameter of the disk surface addressed by the transducer head.
According to a further embodiment of the present invention, an inverse impulse response is determined for each transducer head in a hard disk drive. According to still a further embodiment of the present invention, a PES_RRO is determined for each track on a disk surface for which correction of repeatable runout is desired. According to yet another embodiment of the present invention. an inverse impulse response of a transducer head is determined with respect to each track for which correction of repeatable runout is desired.
Based on the foregoing summary, a number of salient features of the present invention are readily discerned. A method for determining the inverse impulse response of a transducer head in a hard disk drive is provided. The method enables the generation of runout correction values, without requiring the use of a servo track writer to generate those values. In addition, the method provides an inverse impulse response that is particular to the transducer head for which it is derived, and that can be calculated using the controller of the hard disk drive. Furthermore, the present invention provides a hard disk drive having embedded runout correction values that are calculated using an inverse impulse response that is particular to a transducer head in a hard disk drive. The method and the apparatus of the present invention provide improved embedded runout correction, which allows a transducer head to more nearly follow the ideal shape of a track, even though the track has an actual written shape that is not ideal.
Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the following drawings.